Thickness-Dependent Spintronic Terahertz Emission in Molecular Beam Epitaxy-Grown PtTe2: From Semiconductor to Type-II Dirac Semimetal

Abstract

Abstract Spintronic THz emitters are the most practical broadband sources, but their performance is constrained by the spin Hall conductivity of the conversion layer. We show that PtTe2, a type II Dirac semimetal, overcomes this limitation through thickness-driven electronic phase control. Growing PtTe2 films from 1 to 20 monolayers (MLs) by molecular beam epitaxy, we find that THz emission mirrors the electronic phase diagram: it remains weak in the semiconducting single layer, turns on sharply at the semimetal transition near 2 ML, and reaches a maximum at 10 ML. When compared to a reference Pt emitter, this thickness dependence cannot be explained by an intrinsic inverse spin Hall effect but points to a spin-to-charge conversion dominated by the inverse Rashba-Edelstein effect involving topological surface states and Rashba spin splitting. First-principles calculations reproduce this picture quantitatively. These results establish the thickness engineering of van der Waals semimetals as a practical strategy for optimizing spintronic THz emitters and devices.